The invention relates to an especially energy-efficient two-step method and to a system for continuous or semicontinuous production of crystalline calcium carbonate (precipitated calcium carbonate, PCC) by reacting calcium hydroxide with CO2, the calcium hydroxide being milk of lime. Flue gas is exclusively used as CO2 source in the first step of the nucleation and has a CO2 content of 4-25%. In the second step, the complete conversion of the milk of lime reacted in the first step to a maximum of 90%, preferably from 10 to 90%, is operated exclusively using a rich gas which comprises 30 to 99% of CO2, preferably using biogas.
The method according to the invention furthermore effects the simultaneous cleaning of the biogas preferably used which is purified to give biomethane by depletion of the CO2. The biomethane obtained by the method according to the invention preferably still has a CO2 content of from 0.1 to 3%.
Using the method according to the invention and the associated system, the required amount of CaCO3 may always be produced in the desired morphology with especially high energetic efficiency and especially high volumetric productivity in line with demand and independently of fluctuations in the provision of the rich gas, e.g. the biogas, by additional use of flue gas.
According to the prior art, a multiplicity of methods are known in which PCC is formed in aqueous suspension from Ca(OH)2 with introduction of CO2 (usually from flue gases). Thereafter, liquid CO2 or CO2-containing flue gases are introduced into milk of lime using differing ventilation systems and frequently with the aid of additives or seed crystals, the desired morphologies are generated. On an industrial scale, usually the batch operation is employed, some methods of continuous PCC production also being known.
For example, DE 199 43 093 A1 describes a method for the continuous production of CaCO3 which is characterized by a nucleation phase and seed growth phase which are separate from one another. The concentration of the milk of lime and the quantitative ratio of Ca(OH)2/CaCO3 thereafter determine the crystal size. Energetic aspects or the complete utilization of the CO2 contained in the gas stream are not considered.
EP 2 139 831 discloses a method for cleaning biogas, in which CaCO3 is formed simultaneously in crystalline form (PCC) and can be supplied to commercialization. As is known, this method leads to two possibilities for adding value—biomethane which is ready to feed into the grid and PCC in the form of a functional crystal.
Flue gases that are usually used for producing PCC, in contrast to biogas, typically have a markedly lower CO2 content. The production of precipitated calcium carbonate in approaches carried out in standard batehwise methods is energetically demanding, since independently of the selection of the CO2 input system, a large amount of inert gases need to be conjunctionally transported, compressed and introduced. In industrial practice, the lower limit of economic operation with flue gases is considered to be at a CO2 content of these flue gases from about 11% to 15%. At these low CO2 contents of the flue gases, then specific mean energy consumption figures result which are in the range of most conventional gas-introduction apparatuses at values from 170 kWh to 190 kWh per ton of PCC. At a mean energy price of about 0.075 per kWh, the energy factor at this point is already of major importance with approximately 13 to 14 per ton of PCC.
A further disadvantage in the use of flue gases is that they must be cooled to approximately 40° C. before intake into the compressor stage. If the flue gases that are in the temperature range from 80° C. to 110° C. were to be fed uncooled to the compressor stage, after the required compression to typically 0.5 to 0.7 bar, gas temperatures far above 120° C. would result. However, such high temperatures with all known gas-introduction systems, owing to steam formation, lead to cavitation, which leads to a considerably decreased material introduction. Cooling is therefore indispensable, but it is expensive in terms of energy and therefore it is an additional not inconsiderable cost factor for PCC production. In a medium PCC onsite system which is operated with about 25 000 Nm3 of converted flue gas, for cooling a refrigeration output of about 5 MW to 6 MW must be applied, depending on the water vapor content and level of the intake temperature. In cost terms this has an effect of about 0.7 MW per ton of PCC or depending on the type of generation of the refrigeration energy works out at about 6 per ton of PCC (calculated with an average 0.01 per kWhthermal). In the case of simple scalenohedral quality grades in the usage range of fillers, the generator prices are only 100 to 120/t, therefore the cooling costs already at this point represent approximately 5% of the total production costs.